Must

 

Must is a modal verb

Let's look at the different uses of MUST:

MUST

1. To express obligation or duty

This also refers to laws and regulations.

• I must memorize all of these rules about modal verbs.
• People must remain seated until the show is over.
• You must wear a seatbelt at all times.

2. To emphasize the necessity of something

• Humans must have drinking water at least every two days.
• You must give up smoking, it's bad for you.
• We must have a special permit to camp in the national park.
• You must study the last two chapters before the test.
• Plants must have light and water to grow.
• You must drive carefully.

3. Deduction - Sure that something is true (Certainty)

We use this when we don't know but we are certain that it is true (based on evidence).

• Look at all of that snow. It must be really cold outside.
• The ground was wet this morning. It must have rained last night.
• Dinosaurs were very big, they must have eaten a lot.
• It's five in the morning and you still haven't gone to bed? You must be tired!
• Jack must be home. I heard a noise coming from his room.

4. Expresses positive logical assumptions (Must + have + past participle)

• That must have been my mother calling me last night, nobody else has my number.
• He must have won the lottery with the new house and car he has just bought.
• She must have been at home - her car was there.

5. A strong recommendation

Something that is highly recommended (stronger than using should)

• We really must get together for dinner sometime.
• You must see the new Peter Jackson movie, it's fantastic.
• The ice cream here is delicious. You must try some.

Must Summary Chart

Mustn't

The negative is Mustn't which refers to prohibition (negative obligation)

Mustn't = Must not

• You mustn't use your smartphone while you are driving.
• You mustn't get on the subway if you haven't paid for the ride.
• You must not open the gift until it is your birthday.
• We must not tell anyone.

Must vs. Have to

Must can be replaced by Have to with little difference in meaning:

• You have to study. (= you must study)
• He has to finish the report by Friday. (= He must finish the report by Friday)
• They have to resit the test. (= They must resit the test)

Have to is a more informal while Must is mostly used in written orders or instructions.

Also, Must expresses obligation imposed by the speaker while Have to expresses external obligation.

• Teacher: You must complete this essay by Friday
• Student: We have to complete this essay by Friday.

When we are mentioning someone else's obligations, we use Have to.

• John has to quit smoking.

For questions it is more common to use Have to instead of Must (which sounds very formal):

• When do you have to pay finish the report?
• Does he have to take a blood test?

The past tense of Must is Had to:

• I had to pay my speeding ticket yesterday.

Mustn't vs. Don't have to

Be careful with the negative of Must and Have to where they DO have a different meaning. Mustn't is a negative obligation (= it is important that you do NOT do something) while Don't have to is an absence of obligation.

Mustn't = it is prohibited; it is not allowed
Don't have to = no obligation; you are not required to do something, especially if you don't want to.

• You must not drink that. (= it is forbidden to drink that; it is not allowed)
• You don't have to drink that. (= you don't need to drink that but you can if you want)
• You mustn't tell John (= Do not tell John)
• You don't have to tell John (= you can tell John if you want to but it is not necessary)